Carbon nanotubes are a type of material having a structure wherein a graphene sheet is rolled in a tubular manner, and their linear structure has an exceedingly high aspect ratio (see Non-patent document 1). Carbon nanotubes are known to have superb mechanical properties such as strength and flexibility, semiconductor or metal conductivity, and highly stable chemical properties. Arc discharge methods, laser evaporation methods, chemical vapor deposition methods (hereunder referred to as CVD) and the like have been reported as methods for producing carbon nanotubes. CVD, in particular, is a synthesis method noted as being suitable for large-scale synthesis, continuous synthesis and high purification (see Saito, R., Shinohara, H., joint editors, “Carbon Nanotubes: Fundamentals and Applications”, Baifukan Co., Ltd., 2004).
Single-wall carbon nanotubes (hereunder abbreviated as “SWCNT”) have been confirmed to exhibit metal properties and semiconductor properties depending on their rolled forms and their diameters, and they are expected to have applications in electrical and electronic elements. Catalytic CVD (see Non-patent document 2, for example), is the main method for growing nanotubes in synthesis of SWCNT. Catalytic CVD uses metal nanoparticles as catalyst. A gaseous carbon source is supplied while thermally decomposing the carbon source at high temperature and growing nanotubes from the catalyst metal nanoparticles. The catalyst nanoparticles are used in a gas-phase dispersed state for the production (method A). Another method employs the catalyst nanoparticles in a state supported on a substrate (method B). Both method A and method B have advantages and disadvantages.
[Existing SWCNT Production Methods]
FIG. 12 shows an overview of method A using a gas-phase dispersed catalyst. Nanotubes are synthesized by simultaneously blowing the catalyst source and carbon source into an external heated reactor. A typical synthesis method classified as method A is the HiPco method (see Non-patent document 3, for example). This method A effectively utilizes the three-dimensional space of the reactor. However, because the catalyst is entrained with the reactive gas, the residence time of the catalyst in the reactor is short and the catalyst becomes incorporated into the product nanotubes. In addition, because the catalyst nanoparticles have a small size of several nm and undergo rapid aggregation, it is difficult to increase the spatial concentration of the catalyst and the nanotube synthesis rate is about 1 g/day per 1 L reactor volume.
FIG. 13 shows an overview of method B using a substrate-supported catalyst. This method B supports the catalyst on a substrate and supplies the carbon source onto the catalyst to grow nanotubes on the catalyst. The Super Growth method (see Non-patent document 4, for example) is classified as such a method B, and it is a typical synthesis method. This method B allows high-speed growth. High-speed growth of 2.5 mm/10 min, for example, is possible (Non-patent document 4). Moreover, the catalyst is anchored on the substrate, thus minimizing incorporation of the catalyst into the synthesized nanotubes. However, because the reactor can only utilize flat two-dimensional space, utilization of the reactor interior space is inferior to method A.
In addition, a separating step is necessary to separate the synthesized nanotubes. For mass production of nanotubes, it is indispensable for the catalyst-supported substrate to be reusable, but such technology has not yet been established. Several patent documents exist describing the use of particles instead of a substrate for anchoring of the catalyst in method B, with synthesis of the carbon nanotubes using a fluidized bed. For example, Patent document 1 discloses a production apparatus for a tube-like carbon material. There is disclosed therein a fluidized bed reactor that accomplishes continuous production of carbon nanotubes (see paragraph [0007] of Patent document 1).
Another technique for production of carbon nanotubes using a fluidized bed is CoMoCATR. This technique is a method for production of carbon nanotubes by contacting a carbon-containing gas with a catalyst comprising a Group VIII metal such as cobalt (Co) and a Group Vla metal such as molybdenum (Mo), which has been developed by Oklahoma University, U.S. and implemented by Southwest technologies. Patent documents 2-10 are U.S. Patents relating to this technique for production of carbon nanotubes, and are a listing of patents owned by Oklahoma University, U.S.
In these fluidized bed synthesis methods, a catalyst is supported on support particles such as porous silica to synthesize nanotubes, the nanotubes are removed from the fluidized bed apparatus together with the support particles, and the support particles and catalyst are dissolved with an acid or the like to recover the nanotubes. Since the catalyst particle-attached support particles are disposable, while the step of removing the support and catalyst from the nanotubes is complex and the procedure is a batch system with low productivity, the cost of SWCNT is extremely high, at 50,000 yen/g or greater.
[Non-patent document 1] S. Iijima, Nature 354, 56 (1991).
[Non-patent document 2] H. Dai, A. G. Rinzler, P. Nikolaev, A. Thess, D. T. Colbert, and R. E. Smalley, Chem. Phys. Lett. 260, 471 (1996).
[Non-patent document 3] HiPco Method: M. J. Bronikowski, P. A. Willis, D. T. Colbert, K. A. Smith, and R. E. Smalley, J. Vac. Sci. Technol. A 19, 1800 (2001).
[Non-patent document 4] K. Hata, D. N. Futaba, K. Mizuno, T. Namai, M. Yumura, and S. Iijima, Science 306, 1362 (2004).
[Patent document 1] Japanese Unexamined Patent Application Publication No. 2003-286015
[Patent document 2] U.S. Pat. No. 6,333,016, “Method of Producing Nanotubes”
[Patent document 3] U.S. Pat. No. 6,413,487, “Method and Apparatus for Producing Nanotubes”
[Patent document 4] U.S. Pat. No. 6,919,064, “Process and Apparatus for Producing Single-Walled Carbon Nanotubes”
[Patent document 5] U.S. Pat. No. 6,955,800, “Method and Apparatus for Producing Single-Walled Carbon Nanotubes”
[Patent document 6] U.S. Pat. No. 6,962,892, “Metallic Catalytic Particle for Producing Single-Walled Carbon Nanotubes”
[Patent document 7] U.S. Pat. No. 6,994,907, “Carbon Nanotube Product Comprising Single-Walled Carbon Nanotubes”
[Patent document 8] U.S. Pat. No. 7,094,386, “Method of Producing Single-Walled Carbon Nanotubes/Ceramic Composites”
[Patent document 9] U.S. Pat. No. 7,153,903, “Carbon Nanotube-Filled Composites Prepared by In-situ Polymerization”
[Patent document 10] U.S. Pat. No. 7,279,247, “Carbon Nanotube Pastes and Methods of Use”